This invention relates to a motor-driven chain saw with a chain brake device releasable as a back kick brake and as a coasting brake comprising
a tension lever movable with respect to the housing of the motor chain saw which can be moved between a brake position and a ready position,
a release lever movably placed on the tension lever which can take up a coasting brake position and a disengaging position,
a Bowden cable the actuation of which causes a relative movement of the release lever and of the tension lever against the action of the restoring force of a spring member,
a brake which is operatively connected with the release lever in such a way that the saw chain is released only when the tension lever is in the ready position and the release lever in the disengaging position.
Motor chain saws with a so-called back kick brake are known by the prior art since a long time. For these saws, the run of the saw chain is stopped for safety reasons when a dangerous situation takes place. Such a dangerous situation is generally recognized or defined in that a jerk movement takes place with a high acceleration of the saw (back kick). Due to its inertia of mass, the hand-guard releases the brake device for the saw chain. Such a release can also take place directly by manually actuating the hand-guard.
The braking of the saw chain takes place by a brake device generally in such a way that a brake band which winds round a coupling drum is tightened and thus the rotation of the coupling drum is stopped by friction. The coupling drum is connected free of rotation with a chain wheel which drives the saw chain.
The brake device can also be actuated independently from the back kick brake with a supplementary brake device known as a coasting brake in a second way.
The coasting brake is to avoid that the saw chain coasts for a certain time (typically for a few seconds) after having disconnected the drive because of the inertia of mass of the drive. This coasting can cause namely a damage of the saw chain when laying down the device. Moreover, the coasting constitutes an endangering potential for the user of the saw. A coasting brake is useful and necessary independently of the fact if it is a motor saw with an electric motor or an internal combustion engine. The actuation of the coasting brake is generally coupled with the gas pedal to be actuated by the user or with the locking key with which the gas pedal can be stopped. This means that, when pulling through the gas pedal (xe2x80x9cstepping on the acceleratorxe2x80x9d), the coasting brake is disengaged so that the saw chain can rotate. Inversely, when releasing the gas pedal, the brake is immediately actuated and the run of the saw stopped. A coupling of the coasting brake functions similarly with the locking key. The coasting brake is released only when the locking key is pressed.
The aim of this invention is to work out a motor chain saw with the brake device mentioned in the introduction in combination with a supplementary brake device known as a coasting brake. The design priority should be a compact construction and a high safety of function.
This aim is achieved by the characteristics indicated in claim 1.
The brake device of the motor chain saw thus has a construction consisting of a tension lever, a release lever, a Bowden cable and a brake of the type explained in the introduction and known from the prior art. According to the invention, it is characterized in that the tension lever is placed linearly slidable with respect to the housing of the motor chain saw and that the release lever is also placed linearly slidable with respect to the tension lever, the two mentioned movement axles of the displacement being appropriately parallel to each other.
This has the advantage that a compacter construction can be achieved since no space is necessary for providing long lever arms. Moreover, the linearity of the movements allows that the tension lever and the release lever can be placed at least partially in the inside of the spring(s) so that there is a further gain of space. The whole mechanism obtains a longish extension which can be surrounded by a protective housing in a simple way. This reduces the susceptibility of the mechanism to troubles from outside and thus contributes to a higher reliability of the brake system.
In a further development of the invention according to claim 2, the tension lever is pressed by a spring member into the brake position. When the tension lever is in the brake position, the saw chain is not disengaged, i.e. it is braked. Due to the effect of the spring member, the brake device thus tends to take up the safe position in which the saw chain is braked. Furthermore, due to the effect of the spring member, it is guaranteed that already low forces can be sufficient to release the back kick brake since the main energy is made available by the spring member for moving the tension lever.
A brake device of the last mentioned type with a spring member on the tension lever can be further developed according to claim 3 so that the tension lever is held in the ready position by an articulated lever connected with the hand-guard of the motor chain saw against the pressure of the spring member. An articulated lever is an element substantially consisting of two segments, whereby the segments are connected with each other over a hinged articulation. The segments can be moved into an extended position in which the angle of the articulated lever is 1800 or slightly more and a further extension is avoided by a stopper. This extended position of the articulated lever is stable against axial forces of pressure, a relatively low force onto the knee link in vertical direction to the articulated lever axle can however cause the collapse of the articulated lever. The articulated lever can thus be used in the extended position to held the tension lever in the ready position against the high force of the spring member. A relatively low force transmitted by the hand-guard to the articulated lever can then however cause the collapsing of the articulated lever so that the tension lever can move under the action of the elastic force immediately and unhindered from the ready position into the brake position. A stop of the saw chain is thus achieved by the brake.
Moreover, the above described connection of the tension lever with a spring member can be further developed according to claim 4 so that the release lever which is in a disengaging position, fixed to the tension lever but linearly movable is operatively connected with said spring member. For example, the release lever can have a direct contact with the spring member. This construction measure has the advantage that in the disengaging position of the release lever the force of the spring member can be directly transmitted to the release lever. Should it thus come to a release of the back-kick brake with which the force of the spring member is released, this force can act directly and without intermediate elements onto the release lever and thus onto the brake. Additional time delays due to the intermediate elements are avoided here. Furthermore, the safety of the system is increased since the intermediate elements are avoided as possible sources of error.